ABSTRACT

The study of a limited amount of subsurface stratigraphic section associated with the Jurassic-Cretaceous transition in North Dakota has resulted in a new interpretation of the boundary. The Jurassic-Cretaceous contact is postulated to be gradational for the most part rather than at an unconformity. A hypothetical time line, arbitrarily placed by stratigraphic position, is used to demarcate the boundary.

An arbitrary Upper Jurassic time-rock unit, termed Jurassic XB, of varied lithology is used for this report. The XB unit of western North Dakota is correlative with the marine Swift formation of Montana. The XB unit in southwestern North Dakota is more arenaceous and gradually changes to the characteristics of the marine Sundance formation in the northern Black Hills. In south-central North Dakota the XB unit is also correlative with the Sundance formation. However, in the eastern and north-central areas of North Dakota the X B unit changes in lithology and consists of marine sandstones, greenish-gray and gray shales, and light gray siltstones that grade, in an eastern direction, laterally and vertically into partly non-marine quartzose sandstone and gray shale units. It must be emphasized, herein, that the quartzose sandstone and gray shale units have previously been termed, by lithologic criteria only, the Lakota and Fuson formations, respectively. Furthermore, in northeastern North Dakota the XB unit includes gray, greenish gray, and varicolored shales, and light gray silts that are, at least, partly non-marine.

The contact of the XB unit and the underlying Rierdon formation is gradational in northwestern North Dakota and is placed at a prominent point on the electric and/or gamma ray logs. In other parts of North Dakota the contact is at an unconformity. The Rierdon-XB boundary is thought to be a time line, because it is delimited by an unconformity of low angularity in eastern and southern North Dakota and an equivalent widespread "marker horizon" in northwestern North Dakota.

The sediments of the Cretaceous interval studied also show definite facies relationships. The unit overlying the Jurassic XB is termed the Cretaceous YA and consists of fine-to very coarse-grained quartzose sandstones and gray shales of marine to non-marine environments. The Fall River sandstone is, timewise, the only persistent and mappable formation of the YA unit. The units overlying the Cretaceous YA are, in ascending order, the marine Skull Creek shale, the marine to non-marine Newcastle sandstone, and the marine Mowry shale. The upper limit of the stratigraphic interval studied is placed at a prominent point on the gamma ray log, and is at the base of an arenaceous, bentonitic bed that is included in the overlying Belle Fourche shale.

The epeirogenic movement that controlled the deposition of the Jurassic sediments studied can best be described as a gradual uplift with intermittent subsidence. Conversely, the epeirogenic movement that controlled the deposition of the Cretaceous sediments studied can best be described as a gradual subsidence with intermittent uplift.

The lithologies and environments discussed in this report are time transgressive. Tectonics of a regional nature controlled the sedimentation and as a result the lithologies and, therefore, environments of the Jurassic appear to be chiefly time transgressive in a western direction in North Dakota. The overlying Cretaceous lithologies appear to be chiefly time transgressive in an eastern direction. The Jurassic-Cretaceous boundary is then wrapped in a history of regional tectonics. This boundary is arbitrarily placed within a gradational contact in North Dakota, but in other areas along the basinal flanks the Jurassic-Cretaceous boundary certainly appears to be at an unconformity.